The present invention relates to methods and apparatus for determining the imbalance of a rotational member, particularly a wheel, and more specifically to methods and apparatus that are capable of dynamically determining the imbalance of a rotational member. Although it is discussed hereinafter with particular reference to the balancing of wheels, it will be appreciated that aspects of this invention are applicable to other types of rotational members, e.g., rotors or the like. It will be appreciated that when this application refers to balancing "wheels" what is actually being balanced is usually a wheel-tire combination, and that "wheel" includes this combination.
At present, there are available two basic types of systems for determining the imbalance of a wheel having a tire mounted thereon. One of these types of systems is a static one, in which the wheel and tire remain motionless while any imbalance thereof is determined. For example, the wheel can be supported in a horizontal orientation by means of a leveling type of support device having a bubble or similar such indicator that is shifted from a central reference point in dependence upon the imbalance of the tire. By placing suitable weights at strategic points around the rim of the wheel, an operator is able to vary the position of the bubble indicator until it is brought to the reference point. Thereafter, the weights are fixed to the wheel at these strategic points.
The second basic type of wheel balancing system, a dynamic balancing system, has been developed to provide a more complete indication of the imbalance of a wheel and the manner in which it can be corrected. In dynamic wheel balancing systems, the wheel to be balanced is mounted on a shaft that is brought up to a predetermined rotational speed. Once the wheel attains this required speed, measurements are made of the lateral movement of the shaft, or the forces generated by the shaft, due to the imbalance of the wheel. From these measurements, indications of the magnitude of the imbalance of the wheel and the location of the imbalance can be made. While a dynamic balancing system can determine the location of imbalance in a single plane, with this method counterbalancing centrifugal forces present on two planes, generally the inner and outer planes of the wheel, can be detected, distinguished and corrected. Thus, a more precise correction of imbalance is attained. Thereafter the wheel is demounted from the balancer.
Typically, the rotational speed of the wheel is held constant at 400 rpm or greater during dynamic imbalance measurement. Such a speed has been used for several reasons. For one, it is easier to measure the forces of imbalance at high speed as the magnitude of the imbalance forces increases as the square of rotational velocity. At high velocities very favorable signal/noise ratios are easily obtained. For another, there was a generally held but erroneous belief that wheels needed to be balanced at rotational velocities approximating road speed.
U.S. Pat. No. 4,435,982 of Borner and Jackson discloses an improvement in dynamic balancers wherein the rotational velocity of measurement is reduced to less than 360 rpm and the imbalance measurement is carried out with the wheel drive system disconnected and the wheel in a "coasting" mode.
U.S. Pat. No. 4,507,964 of Borner and Jackson describes a further improvement in which the required rotational velocity is further reduced to 100 rpm or even lower and the wheel is rotated manually and permitted to coast. The just noted Borner-Jackson patents are incorporated herein by reference.
Machines embodying the Borner-Jackson patents are being marketed to the trade under the trademarks Balco 80 and Balco 90 respectively. The Balco 80 and Balco 90 balancers are characterized by carrying out their imbalance measurement when the rotational speed has coasted down to equal any one of one or several predetermined measuring velocities. The Balco 90 machine performs its function particularly well in this regard. However, at times with unexperienced operators, the wheel will be hand spun substantially in excess of the required preset measuring velocity and a time-consuming wait for the wheel to slow to that velocity is incurred.
The Balco machines, and to our knowledge other dynamic balancers, have the characteristic that the carrier for the rotatable shaft upon which the wheel is mounted is fixedly attached to a base or pedestal. The wheel is manually lifted up onto the shaft and mounted so it will clear the floor and spin. This is a simple configuration that is quite convenient when the wheels to be balanced are motorcycle or light automobile wheels. However, when the wheels are large and heavy, i.e., large automobile wheels or truck wheels, the positioning and mounting becomes difficult and dangerous. A separate lifting device, such as a jack or fork lift is usually required with two persons often being needed to position the wheel and mount it. This greatly increases the time and costs involved in balancing these large heavy wheels.
Another problem which occurs when balancing large and heavy wheels is that the wheels are not easily portable and it would often be preferable to be able to move the balancer to the wheel rather than vice versa. The above-noted Balco 90 machine is portable but other balancing machines used heretofore are large pieces of equipment generally requiring permanent in-shop installation.